In the 1950s, the 7.62×51mm NATO rifle cartridge (physically interchangeable with, but not identical to, the .308 Winchester rifle cartridge)[4] was selected to replace the .30-06 Springfield as the standard NATO rifle cartridge. At the time of selection, there had been criticism that the 7.62×51mm NATO was too powerful for lightweight modern service rifles, causing excessive recoil, and that as a result it did not allow for sufficient automatic rate of fire from hand-held weapons in modern combat.[citation needed]

The British had extensive evidence with their own experiments into an intermediate cartridge since 1945 and were on the point of adopting a .280 inch (7 mm) cartridge when the selection of the 7.62×51mm NATO was made. The FN company had also been involved in the development of the .280 round including developing a version of the FN FAL in .280.[5] The concerns about recoil and effectiveness were effectively overruled by the US within NATO, and the other NATO nations accepted that standardization was more important at the time than selection of the ideal cartridge.([6]The EM-2, Rifle No.9 Mk1 or "Janson rifle", was an experimental British assault rifle briefly adopted by British forces in 1951, but the decision was overturned very shortly thereafter by Winston Churchill's incoming government in an effort to secure NATO standardisation of small arms and ammunition in the face of American intransigence.) However, while the 7.62×51mm NATO round became NATO standard, the US was already engaged in research of their own, which ultimately led to the 5.56×45mm NATO cartridge.[citation needed]

During the late 1950s, ArmaLite and other U.S. firearm designers started their individual Small Caliber/High Velocity (SCHV) assault rifle experiments using the commercial .222 Remington cartridge. When it became clear that there was not enough powder capacity to meet U.S. Continental Army Command's (CONARC) velocity and penetration requirements, ArmaLite contacted Remington to create a similar cartridge with a longer case body and shorter neck. This became the .222 Remington Special. At the same time, Springfield Armory's Earle Harvey had Remington create an even longer cartridge case then known as the .224 Springfield. Springfield was forced to drop out of the CONARC competition, and thus the .224 Springfield was later released as a commercial sporting cartridge known as the .222 Remington Magnum. To prevent confusion among all of the competing .222 cartridge designations, the .222 Remington Special was renamed the .223 Remington. With the U.S. military adoption of the ArmaLite M16 rifle in 1963, the .223 Remington was standardized as the 5.56×45mm NATO. As a commercial sporting cartridge the .223 Remington was introduced in 1964.

The 5.56×45mm cartridge, along with the M16 rifle, were initially adopted by U.S. infantry forces as interim solutions to address the weight and control issues experienced with the 7.62×51mm round and M14 rifle. In the late 1950s, the Special Purpose Individual Weapon program sought to create flechette rounds to allow troops to fire sabot-type projectiles to give a short flight time and flat trajectory with a muzzle velocity of 1,200 metres per second (3,900 ft/s) to 1,500 metres per second (4,900 ft/s). At those speeds, factors like range, wind drift, and target movement would no longer affect performance. Several manufacturers produced varying weapons designs, including traditional wooden, bullpup, "space age," and even multi-barrel designs with drum magazines. All used similar ammunition firing a 1.8 mm diameter dart with a plastic "puller" sabot filling the case mouth. While the flechette ammo had excellent armor penetration, there were doubts about their terminal effectiveness against unprotected targets. Conventional cased ammunition was more accurate and the sabots were expensive to produce. The SPIW never created a weapons system that was combat effective, so the M16 was retained, and the 5.56 mm round was kept as the standard U.S. infantry rifle cartridge.[5]

In a series of mock-combat situations testing in the early 1960s with the M16, M14 and AK-47, the Army found that the M16's small size and light weight allowed it to be brought to bear much more quickly.[citation needed] Their final conclusion was that an 8-man team equipped with the M16 would have the same fire-power as a current 11-man team armed with the M14.[citation needed] U.S. troops were able to carry more than twice as much 5.56×45mm NATO ammunition as 7.62×51mm NATO for the same weight, which would allow them a better advantage against a typical NVA unit armed with AK-47, AKM or Type 56 assault rifles.

In 1977, NATO members signed an agreement to select a second, smaller caliber cartridge to replace the 7.62×51mm NATO cartridge.[9] Of the cartridges tendered, the 5.56×45mm NATO was successful, but not the 55 gr M193 round used by the U.S. at that time. The wounds produced by the M193 round were so devastating that many[10] consider it to be inhumane.[11][12] Instead, the Belgian 62 gr SS109 round was chosen for standardization. The SS109 used a heavier bullet with a steel tip and had a lower muzzle velocity for better long-range performance, specifically to meet a requirement that the bullet be able to penetrate through one side of a steel helmet at 600 meters. This requirement made the SS109 (M855) round less capable of fragmentation than the M193 and was considered more humane.[13]

The 5.56×45mm NATO inspired an international tendency towards relatively small-sized, lightweight, high-velocity military service cartridges that produce relatively low bolt thrust and free recoil impulse, favoring lightweight arms design and automatic fire accuracy. Similar intermediate cartridges were developed and introduced by the Soviet Union in 1974 (5.45×39mm) and by the People's Republic of China in 1987 (5.8×42mm).[5][14]

According to the official NATO proofing guidelines the 5.56×45mm NATO case can handle up to 430 MPa (62,366 psi) piezo service pressure. In NATO regulated organizations every rifle cartridge combo has to be proofed at 125% of this maximum pressure to certify for service issue. This is equal to the C.I.P. maximum pressure guideline for the .223 Remington cartridge, which is the 5.56×45mm NATO parent cartridge.

The 5.56×45mm NATO SS109/M855 cartridge (NATO: SS109; U.S.: M855) with standard 62 gr. lead core bullets with steel penetrator will penetrate approximately 38 to 51 cm (15 to 20 in) into soft tissue in ideal circumstances. As with all spitzer shaped projectiles it is prone to yaw in soft tissue. However, at impact velocities above roughly 762 m/s (2,500 ft/s), it may yaw and then fragment at the cannelure (the crimping groove around the cylinder of the bullet).[17] These fragments can disperse through flesh and bone, inflicting additional internal injuries.[18]

Fragmentation, if and when it occurs, imparts much greater damage to human tissue than bullet dimensions and velocities would suggest. This fragmentation effect is highly dependent on velocity, and therefore barrel length: short-barreled carbines generate less muzzle velocity and therefore lose wounding effectiveness at much shorter ranges than longer-barreled rifles.[19] Proponents of the hydrostatic shock theory contend that the rapid transfer of energy also results in wounding effects beyond the tissue directly crushed and torn by the bullet and fragments.[20][21] These remote wounding effects are known as hydrostatic shock.[22]

SS109/M855 NATO ball can penetrate up to 3 mm (0.12 in) of steel at 600 meters.[23] According to Nammo, a Norwegian ammunition producer, the 5.56×45mm NATO M995 armour piercing cartridge can penetrate up to 12 mm (0.47 in) of RHA steel at 100 meters.[24]

The US Army's Ballistic Research Laboratory measured a ballistic coefficient (G7 BC) of 0.151 and form factor (G7 i) of 1.172 for the SS109/M855 ball projectile.[25]

The Swedish military has measured the bullet velocities of SS109/M855 military cartridges at 4 m (13.1 ft) from the muzzle fired from differing barrel lengths:[26]

There has been much debate of the allegedly poor performance of the bullet on target, especially the first-shot kill rate when the muzzle velocity of the firearms used and the downrange bullet deceleration do not achieve the minimally required terminal velocity of over 750 m/s (2,500 ft/s) at the target to cause fragmentation. Many complaints were reported during the Gulf War, Somalia, and in the conflicts in Iraq and Afghanistan. This can be attributed to the change in barrel length, as this cartridge was designed to achieve maximum performance in a 20-inch (51 cm) barrel. The 14.5-inch (37 cm) barrel of the U.S. military's M4 carbine generates considerably less muzzle velocity than the longer 20-inch (51 cm) barrel found on the M16 rifle, and terminal performance can be a particular problem with the M4.

The 5.56×45mm NATO standard SS109/M855 cartridge was designed for maximum performance when fired from a 508 mm (20.0 in) long barrel, as was the original 5.56 mm M193 cartridge. Experiments with longer length barrels up to 610 mm (24.0 in) resulted in no improvement or a decrease in muzzle velocities for the SS109/M855 cartridge. Shorter barrels produce a greater flash and noise signature, and the addition of a suppressor to a short barreled AR family rifle can make it unreliable, as the reduced time for the propellant to burn in the barrel and higher muzzle pressure levels at the suppressor entrance can cause faster cycling and feeding issues. Unless the gas port can be regulated or adjusted for higher pressures, suppressors for short barreled 5.56×45mm NATO firearms must be larger and heavier than models for standard length rifles to function reliably. SS109/M855 cartridges fired from barrels under about 254 mm (10.0 in) in length do not have enough muzzle velocity energy to cause bullet fragmentation that occurs only at terminal velocities of over 750 m/s (2,500 ft/s) on impact, reducing the wounding capacity.[27][28]

Combat operations the past few months have again highlighted terminal performance deficiencies with 5.56×45mm 62 gr. M855 FMJ. These problems have primarily been manifested as inadequate incapacitation of enemy forces despite them being hit multiple times by M855 bullets. These failures appear to be associated with the bullets exiting the body of the enemy soldier without yawing and fragmenting.

This failure to yaw and fragment can be caused by reduced impact velocities as when fired from short barrel weapons or when the range increases.

Although all SS109/M855 types must be 62 gr. FMJ bullets constructed with a steel penetrator in the nose, the composition, thickness, and relative weights of the jackets, penetrators, and cores are quite variable, as are the types and position of the cannelures. Because of the significant differences in construction between bullets within the SS109/M855 category, terminal performance is quite variable – with differences noted in yaw, fragmentation, and penetration depths. Luke Haag's papers in the AFTE Journal (33(1):11–28, Winter 2001) also describes this problem.

If 5.56 mm bullets fail to upset (yaw, fragment, or deform) within tissue, the results are relatively insignificant wounds. This is true for all 5.56×45mm bullets, including both military FMJ and OTM (open tip match) and civilian JHP/JSP designs used in law enforcement. As expected, with decreased wounding effects, rapid incapacitation is unlikely: enemy soldiers may continue to pose a threat to friendly forces and violent suspects can remain a danger to law enforcement personnel and the public.

This failure of 5.56×45mm NATO bullets to yaw and fragment can be caused by reduced impact velocities as when fired from short-barreled weapons or when the range to the target increases. Failure to yaw and fragment can also occur when the bullets pass through only minimal tissue, such as a limb or the chest of a thin, small statured individual, as the bullet may exit the body before it has a chance to yaw and fragment. Two other yaw issues: Angle-of-Attack (AOA) variations between different projectiles, even within the same lot of ammo, as well as Fleet Yaw variations between different rifles, were elucidated in 2006 by the Joint Service Wound Ballistic Integrated Product Team (JSWB-IPT), which included experts from the military law enforcement user community, trauma surgeons, aero ballisticians, weapon and munitions engineers, and other scientific specialists. These yaw issues were most noticeable at close ranges and were more prevalent with certain calibers and bullet styles—the most susceptible being 5.56×45mm NATO FMJ ammunition like SS109/M855 and M193.

Despite complaints that the 5.56×45mm NATO round lacks stopping power, others contend that animal studies of the wounding effects of the 5.56×45mm NATO round versus the 7.62×39mm have found that the 5.56×45mm NATO round is more damaging, due to the post-impact behavior of the 5.56 mm projectile resulting in greater cavitation of soft tissues.[31] The US Army contended in 2003 that the lack of close range lethality of the 5.56×45mm NATO was more a matter of perception than fact. With controlled pairs and good shot placement to the head and chest, the target was usually defeated without issue. The majority of failures were the result of hitting the target in non-vital areas such as extremities. However, a minority of failures occurred in spite of multiple hits to the chest.[32]

Some have contended that shot placement is the most important parameter in determining the lethality of a bullet. Difficulty with the 5.56×45mm NATO at range has been attributed to training, as few nations other than the US[33] teach shooting beyond 200–300 meters to regular soldiers; Swedish ISAF units relied on .50 BMG heavy machine guns for long-range shooting due to resilience to factors such as range and wind drift. Underperformance is thus attributed to errors in range and wind estimation, target lead, firing position, and stress under fire, factors that can be resolved through training.[26][34]

Advances have been made in 5.56 mm ammunition. The U.S. military had adopted for limited issue a 77-grain (5.0 g) "Match" bullet, type classified as the Mk 262. The heavy, lightly constructed bullet fragments more violently at short range and also has a longer fragmentation range.[35] Originally designed for use in the Mk 12 SPR, the ammunition has found favor with special forces[36] units who were seeking a more effective cartridge to fire from their M4A1 carbines. Commercially available loadings using these heavier (and longer) bullets can be prohibitively expensive and cost much more than military surplus ammunition. Additionally, these heavy-for-caliber loadings sacrifice some penetrative ability compared to the M855 round (which has a steel penetrator tip). Performance of 5.56×45mm military ammunition can generally be categorized as almost entirely dependent upon velocity in order to wound effectively. Heavy OTM bullets enhance soft tissue wounding ability at the expense of hard-target/barrier penetration.

U.S. Special Forces had sought to create a round that had increased power out of carbine M4 barrels and compact SCAR-L barrels, while increasing hard target performance. Developmental efforts led to the creation of the Mk318. The bullet uses an open-tip design to inflict damage on soft tissue, and has a brass rear to penetrate hard targets. The tip and lead core fragments consistently even when using short barrels, while the rear moves through once the front impacts.[37] It has more consistent performance because it is not yaw-dependent like the M855; the nose fragments upon impact and solid rear penetrator continues to move relatively straight. This makes the Mk318 effective against personnel with or without body armor. The round also increases accuracy, from 3–5 minute of angle (MOA) with the M855 from an M4A1 barrel to 1.71 MOA at 300 yards and 1.67 MOA at 600 yards from a 14 in (360 mm) SCAR-L barrel.[38]

For general issue, the U.S. Army adopted the M855A1 round in 2010 to replace the M855. The primary reason was pressure to use non-lead bullets. The lead slug is replaced by a copper alloy slug in a reverse-drawn jacket, with a hardened steel penetrator extending beyond the jacket, reducing lead contamination to the environment. The M855A1 offers several improvements other than being lead-free. It is slightly more accurate, has better consistency of effect in regards to wounding ability, and has an increased penetrating capability. The round can better penetrate steel, brick, concrete, and masonry walls, as well as body armor and sheet metal. It penetrates 3⁄8 in (9.5 mm) of mild steel at 350 meters, which the M855 can only do at 160 meters. The propellant burns faster, which decreases the muzzle flash and gives a higher muzzle velocity, an important feature when fired from a short barreled M4 carbine. Though the M855A1 is more expensive to produce, its performance is considered to compensate. One possible danger is that it generates much greater pressure in the chamber when fired, decreasing service life of parts, and increasing the risk of catastrophic failure of the weapon, though this has yet to occur.[39][40]

The U.S. Marines adopted the Mk318 in early 2010 due to delays with the M855A1. This was a temporary measure until the M855A1 was available for them, which occurred in mid-2010 when the Army began to receive the rounds. Both the Mk318 and M855A1 weigh the same and have similar performance, and both have better performance than the M855 against all targets. SOCOM spent less money developing the Mk318 and it is marginally better than the M855A1 in some situations, but costs more per round. The Army spent more developing the M855A1 which performs as well or nearly as well as the Mk318, but is cheaper per round and has the advantage of being lead-free. While SOCOM constantly looks for better equipment, the Army and Marines have far more troops to supply and buy more ammunition than SOCOM.[41]

If the 5.56 mm bullet is moving too slowly to reliably fragment on impact, the wound size and potential to incapacitate a person is greatly reduced. There have been numerous attempts to create an intermediate cartridge that addresses the complaints of 5.56 NATO's lack of stopping power along with lack of controllability seen in rifles firing 7.62 NATO in full auto. Some alternative cartridges like the .270 caliber 6.8mm Remington SPC (6.8×43mm) focused on superior short-range performance by sacrificing long-distance performance due to the relatively short engagement distances typically observed in urban warfare. Others, like the 6.5mm Grendel (6.5×39mm), are attempts at engineering an all purpose cartridge that could replace both the 5.56 and 7.62 NATO rounds. The 300 AAC Blackout (7.62×35mm) round was designed to have the power of the 7.62×39mm for use in an M4 platform using standard M4 magazines, and to easily interchange between subsonic ammunition for suppressed firing and supersonic rounds. All these cartridges have certain advantages over the 5.56×45mm NATO, but they have their own individual tradeoffs to include lower muzzle velocity, less range, decreased magazine capacity, and different internal parts. None of these cartridges have gained any significant traction beyond sport shooting communities.

By late 2004, the 6.8mm Remington SPC was in limited use with U.S. Special Operators.[42] However, it was not adopted for widespread use due to resistance from officials on changing calibers.[43] In 2007, both the U.S. SOCOM and the U.S. Marine Corps decided not to field weapons chambered in 6.8×43mm due to logistical and cost issues.[44] An unnamed LWRC representative said in January 2014 that the US military is once again taking a look at the 6.8 SPCII after all the commercial development in the last 10 years.[45]

The 5.56 mm NATO and .223 Remington cartridges and chamberings are similar but not identical.[46] While the cartridges are identical other than powder load, the chamber leade, i.e. the area where the rifling begins, is cut to a sharper angle on some .223 commercial chambers. Because of this, a cartridge loaded to generate 5.56mm pressures in a 5.56mm chamber may develop pressures that exceed SAAMI limits when fired from a short-leade .223 Remington chamber.

The dimensional specifications of 5.56 NATO and .223 commercial brass cases are identical. The cases tend to have similar case capacity when measured, with variations chiefly due to brand, not 5.56 vs .223 designation. The result of this is that there is no such thing as "5.56 brass" or ".223 brass", the differences in the cartridges lie in pressure ratings and in chamber leade length, not in the shape or thickness of the brass.[47][48]

In July 2012, the Army solicited a request for vendors to supply alternative cartridge cases to reduce the weight of an M855A1 5.56 mm round by at least 10 percent, as well as for the 7.62 NATO and .50 BMG rounds. The cartridge cases must maintain all performance requirements when fully assembled, be able to be used by the Lake City Army Ammunition Plant, must be manufactured in quantities totaling approximately 45 million per year. Polymer-cased ammunition is expected as a likely lightweight case technology.[49] A hybrid polymer/metal version of a conventional cartridge case would be thicker than regular cases and reduce the amount of space for the propellant,[50] although certain polymers could be thermodynamically more efficient and not lose energy to the case or chamber when fired.[51]

C.I.P. defines the maximum service and proof test pressures of the .223 Remington cartridge equal to the 5.56 mm NATO, at 430 MPa (62,366 psi). This differs from the SAAMI maximum pressure specification for .223 Remington of 380 MPa (55,114 psi), due to CIP test protocols measuring pressure using a drilled case, rather than an intact case with a conformal piston, along with other differences.[52] NATO uses NATO EPVAT pressure test protocols for their small arms ammunition specifications.

Because of these differences in methodology, the CIP pressure of 430 MPa (62,366 psi) is the same as a SAAMI pressure of 380 MPa (55,114 psi), which is reflected in US Military specifications for 5.56 mm NATO, which call for a mean maximum pressure of 55,000 PSI (when measured using a protocol similar to SAAMI).[53]

These pressures are generated and measured using a chamber cut to 5.56 NATO specifications, including the longer leade. Firing 5.56mm NATO from a chamber with a shorter .223 Remington leade can generate pressures in excess of SAAMI maximums.

The 5.56 mm NATO chambering, known as a NATO or mil-spec chamber, has a longer leade, which is the distance between the mouth of the cartridge and the point at which the rifling engages the bullet. The .223 Remington chambering, known as SAAMI chamber, is allowed to have a shorter leade, and is only required to be proof tested to the lower SAAMI chamber pressure. To address these issues, various proprietary chambers exist, such as the Wylde chamber (Rock River Arms)[54] or the ArmaLite chamber, which are designed to handle both 5.56×45mm NATO and .223 Remington equally well. The dimensions and leade of the .223 Remington minimum C.I.P. chamber also differ from the 5.56 mm NATO chamber specification.

Using commercial .223 Remington cartridges in a 5.56 mm NATO chambered rifle should work reliably, but until recently, it was believed this was less accurate than when fired from a .223 Remington chambered gun due to the longer leade.[55] Although that may have been true in the early 1960s when the two rounds were developed, recent testing has shown that with today's ammunition, rifles chambered in 5.56mm can also fire .223 ammunition every bit as accurately as rifles chambered in .223 Remington, and the 5.56mm chamber has the additional advantage of being able to safely fire both calibers.[56] Using 5.56 mm NATO mil-spec cartridges (such as the M855) in a .223 Remington chambered rifle can lead to excessive wear and stress on the rifle and even be unsafe, and SAAMI recommends against the practice.[57][58] Some commercial rifles marked as ".223 Remington" are in fact suited for 5.56 mm NATO, such as many commercial AR-15 variants and the Ruger Mini-14 (marked ".223 cal", except the Mini-14 "Target" model, which only fires .223), but the manufacturer should always be consulted to verify that this is acceptable before attempting it, and signs of excessive pressure (such as flattening or gas staining of the primers) should be looked for in the initial testing with 5.56 mm NATO ammunition.[59]

It should also be noted that the upper receiver (to which the barrel with its chamber are attached) and the lower receiver are entirely separate parts in AR-15 style rifles. If the lower receiver has either .223 or 5.56 stamped on it, it does not guarantee the upper assembly is rated for the same caliber, because the upper and the lower receiver in the same rifle can, and frequently do, come from different manufacturers – particularly with rifles sold to civilians or second-hand rifles.

In more practical terms, as of 2010[update] most AR-15 parts suppliers engineer their complete upper assemblies (not to be confused with stripped uppers where the barrel is not included) to support both calibers in order to satisfy market demand and prevent any potential problems.

Benefits of the 5.56 NATO claimed over the 7.62 NATO include equal lethality, half the mass and volume, reduced recoil and signature, better penetration of metal plates, flatter trajectory and shorter time of flight out to 700 meters, weapons chambered for it are lighter, and better hit probability. Hit probability refers to the ability of a soldier to concentrate on firing in spite of their weapon's recoil and noise, which is noticeably different between the two cartridges. The 7.62 NATO has twice the impact energy of the 5.56 NATO which is needed if a target is protected by armor, but if not both rounds normally penetrate through enemies past 600 meters. A 5.56 NATO round fired from a 20 in (510 mm) barrel has a flatter trajectory than a 7.62 NATO round fired from a barrel of equal length, while the 5.56 NATO fired from a 14.5 in (370 mm) barrel has the same trajectory as the 7.62 NATO from a 20 in barrel, as well as the same time of flight. A 7.62 NATO round reaches 50 percent of its velocity within 80 mm (3.1 in) of the barrel when fired, so decreasing the barrel length for close quarters combat results in increased muzzle pressure and greater noise and muzzle flash.[26][34]

5,6mm Gw Pat 90: The 63-grain 5.56 x 45mm Gewehrpatrone 90 / 5,6mm GP 90 ("5.6-mm Rifle Cartridge 90") is the Swiss Army's standard 5.56mm Ball round. It is optimized for use with the Sturmgewehr 90 service rifle, both which were adopted in 1987. Originally the cartridge had a cupro-nickel jacketed bullet and Berdan primer, but it now has a tombac jacketed bullet and lead-free Boxer primer. Since 1997 most components of the round are made in Switzerland.

Cartridge, Caliber 5.56 mm, High Pressure Test (HPT), M197 [Crimped tip with stannic-stained or nickel-plated case]: High-pressure Testing Blank used when proofing weapons during manufacture, test, or repair. Warning: do not use as a training blank or grenade-launching blank.

Cartridge, Caliber 5.56 mm, Ball, M855 Lead Free [Green tip]: 62-grain bullet with a steel penetrator tip over a tungsten-composite core in a partial copper jacket. Primarily used during training in countries with strict lead disposal laws.[68]

Cartridge, Caliber 5.56 mm, Plastic, Practice, M862 [Brass primer, Aluminum case and Blue plastic projectile]: Short Range Training Ammo (SRTA) has a smaller charge than standard ball, reducing its aimed range to 250 meters, and fires a plastic bullet. The M2 training bolt must be used in the M16 Rifle / M4 Carbine when using SRTA for the weapon to cycle properly due to its lower power. It is used during training on shooting ranges near built-up or populated areas.

Cartridge, Caliber 5.64 mm, Ball, MLU-26/P(National Stock Number:1305-968-5892): Munition, Live, Unit / Personnel Use. Early USAF designation for a 55-grain 5.56×45mm FMJ ball cartridge produced by Remington-Union Metallic Cartridge Company (headstamp RA 63 or REM-UMC 63) in 1963. It was their designation for the commercial 55-grain Remington .223 M.C. ("Metallic-Cased", or Full Metal Jacketed) cartridge, which the Air Force initially designated "5.64 mm" rather than 5.56 mm. The order consisted of 8.5 million rounds and was procured for testing, training and unconventional warfare use with the XM16 rifle.

In 1970, NATO decided to standardize a second rifle caliber. Tests were conducted from 1977 to 1980 using U.S. XM177 5.56 mm, Belgian SS109 5.56 mm, British 4.85×49mm, and German 4.7x33mmcaseless. No weapon could be agreed upon, as many were prototypes, but the SS109 was found to be the best round and standardized on October 28, 1980. The SS109 was developed in the 1970s for the FN FNC rifle and the FN Minimi machine gun. To increase the range of the Minimi, the round was created to penetrate 3.5 mm of steel at 600 meters. The SS109 had a steel tip and lead rear and was not required to penetrate body armor. Barrels required at least a 1:9 in rifle twist, but needed a 1:7 in rifle twist to fire tracer ammunition.[26][34][38] The U.S. designated the SS109 cartridge the M855 and first used it in the M16A2 rifle. The 62-grain round was heavier than the previous 55-grain M193. While the M855 had better armor penetrating ability, it is less likely to fragment after hitting a soft target. This lessens kinetic energy transfer to the target and reduces wounding capability.[74] The M855 is yaw dependent, meaning it depends on the angle upon which it hits the target. If at a good angle, the round turns as it enters soft tissue, breaking apart and transferring its energy to what it hits. If impacting at a bad angle, it could pass through and fail to transfer its full energy.[40] The SS109 was made to pierce steel helmets at long range from the Minimi, not improve terminal performance on soft tissue from rifles or carbines.[30] In Iraq, troops that engaged insurgents at less than 150 yards found that M855 rounds did not provide enough stopping power. In addition to not causing lethal effects with two or more rounds, they did not effectively penetrate vehicle windshields, even with many rounds fired at extremely close range.[75] In Afghanistan, troops found that M855 rounds also suffered at long ranges. Although 5.56 mm rifles have an effective range of 450–600 meters, the M855 bullet's performance falls off sharply beyond 300 meters. The ranges are even shorter for short-barreled carbines. Half of small-arms attacks were launched from 300–900 meter ranges.[76] An M855 fired from an M4 Carbine has severely degraded performance beyond 150 meters.[30]

The maximum effective point target range of an M4 carbine with M855 rounds is 500 meters, with a maximum effective area target range of 600 meters. These mark the greatest distances the rounds can be expected to accurately hit the target, not the ranges that they have terminal effectiveness against them. Because the M855 is yaw dependent it requires instability in flight to deform upon hitting the target. It is the most stable in flight between 150–350 meters, potentially lessening its effectiveness if it strikes an enemy between those distances. In addition to this, tests have shown that 5.56 mm bullets fragment most reliably when traveling faster than 2,500 ft/s (760 m/s). From full-length 20 in rifle and machine gun barrels, rounds are kept above this velocity out to 200 meters. An M855 from an M4 has a muzzle velocity of 2,970 ft/s (910 m/s), but that is reduced to 2,522 ft/s (769 m/s) by 150 meters. Even if it impacts at optimum speeds, 70 percent of 5.56 mm bullets will not begin to yaw until 4.7 in (120 mm) of tissue penetration. 15 percent more begin to yaw after that distance, so up to 85 percent of rounds that hit do not start to fragment until nearly 5 in of penetration. Against small statured or thin combatants, the M855 has little chance of yawing before passing through cleanly and leaving a wound cavity no bigger than the bullet itself. The factors of impact angle and velocity, instability distance, and penetration before yaw reduce the round's predictable effectiveness considerably in combat situations.[77]

In mid-2005, a meeting was held at the Lake City Army Ammunition Plant as part of Phase II of the U.S. Army's Green Ammunition replacement program and attended by representatives from several military sectors and contractors. The objective was to create an environmentally-friendly small arms training round after the Army had been required, in part by stronger state regulations, to remove lead from their bullets to reduce lead accumulation at stateside training ranges. Initially, Phase I efforts created the M855 "green tip" 5.56 mm round that replaced the lead core with one made of tungsten, but it was found that the round would become unstable in flight and fly sideways through a target; furthermore, research was suggesting that tungsten was not environmentally better than lead. Phase II efforts focused on creating an alternative round that did not contain either metal. As the Iraq War was happening at the same time, participants decided to use the opportunity to redirect their efforts from just making a "green" training round to creating an entirely new general-purpose rifle round to address complaints encountered in the field.[78]

Complaints had surfaced about the poor lethality of standard M855 rounds against soft targets, although while some units claimed they had virtually no effect others said they were having no problems. The problem was "yaw sensitivity," where rounds experiencing swing in flight have varying effects when they hit a target based on the degree of yaw, resulting in unpredictable lethality effects. The makeup and design of the bullet affected accuracy. The copper jacket is formed first at the point, then a steel penetrator is inserted into the jacket and the projectile is crimped shut. Error in the penetrator placement can change flight performance and affect dispersion, and the rearward-drawn jacket can create an uneven boat tail leading to uneven airflow and more affected dispersion. It also was not optimized for use in short-barreled rifles. The M855 was originally made to burn and accelerate from the 20 in (510 mm) barrel of the M249 SAW; then it was repurposed to the M16A2 for ammunition compatibility. When fired from a short-barreled M4, lower muzzle velocity led to decreased terminal effectiveness, and unburned propellant fired from the end of the barrel created a brighter muzzle flash and caused more combustion debris to be pushed back in the gas tube, dirtying the weapon.[78]

Alternative 5.56 mm rounds in use were considered, like the Mk 262 open-tipped match cartridge. The Mk 262 was more accurate and had better soft target performance, and both Army and Marine units were using large quantities in Iraq deployments for designated marksman rifles. However, the Mk 262 was not adequate as a general-purpose round and cost four times more than the M855, so it could not be produced at the volumes required for 5.56 mm ammunition (up to 1 billion rounds annually). It was also found that performance suffered when fired from standard barrels, so match grade barrels would be needed for optimum performance. The M995 armor-piercing round had high accuracy and target penetration, but was eight times more expensive than the M855. Commercial alternatives were found to have the same yaw sensitivity issues as the M855.[78]

A number of objectives were desired for the new round, including more lethal, or at least more consistent, soft target effects and less shot dispersion. It must not be specialized for any one task to have general-purpose performance, and if improvement in any one area resulted that would be considered a secondary benefit. As the services were purchasing large numbers of optics, it could not require a different reticle pattern, but re-zeroing sights would be acceptable. Optimization for short-barreled rifles was needed, as the Army and Special Forces' main weapon was the M4 carbine; a flash-suppressed propellant was also required. Optimizing the round to perform within the pressure limits for short barrels would cause performance loss in longer barrels (causing complaints from the Marine Corps), but analysis showed that a short barrel-optimized round would have less performance loss fired from a long barrel than vice versa. The round also had to be cost-competitive and meet environmental standards.[78]

The initial M855A1 design began testing in late-2007. Its alloy core had no toxic metals and could be cheaply produced. Materials and production methods, like a reverse-drawn jacket for a clean boat tail, led to yaw insensitivity, improved soft target performance, and consistent dispersion. The steel penetrator was retained and, through greater velocity and better composition and placement, substantially improved hard target performance. It met short-barrel optimization, ballistic similarity, and producibility requirements; it cost slightly more than the M855, but far less than the Mk 262. Production prove-out testing would take time, as over a million rounds had to be test-fired to ensure instances of failure would not occur at the 1 billion rounds per year scale. As information began to be made public, the original program objective to create a "green round" led to confusion and criticism that the Army was focusing more on environmental safety than on performance. Just as testing was being completed and the first production lots were being prepared in 2009, the rounds were found to fail at high temperatures. This caused a one-year delay to replace the bullet core, which solved the temperature problem and also improved cost and producibility.[78]

In December 2014, small ammunition company Liberty Ammunition won a $15.6 million lawsuit against the Army for passing on proprietary data and specifications for its lead-free copper-core, steel-tipped bullet design to other vendors. The company founder presented his idea for an enhanced performance incapacitative composite (EPIC) 5.56 mm round to Army officials and gave technical and performance data to SOCOM in 2005, during Phase II of the green ammunition program, after a non-disclosure agreement was signed protecting proprietary information. Liberty Ammunition was formed to produce the EPIC round and received a SOCOM contract to test it, completed in 2007. The round's design patent was approved in 2010, the same year the M855A1 was fielded. A suit was filed claiming the Army shared confidential information to potential vendors, and the U.S. Court of Federal Claims found that the Army had violated three non-disclosure agreements. The court also ordered the Army to pay 1.4 cents to the company for every M855A1 round produced until its patent expires in 2027, totally between $2.2 million to $5.3 million per year.[79]

On June 24, 2010, the United States Army announced it began shipping its new 5.56 mm cartridge, the M855A1 Enhanced Performance Round (EPR), to active combat zones. During testing, the M855A1 performed better than M80 7.62×51mm NATO ball ammunition against certain types of targets (particularly hardened steel), blurring the performance differences that previously separated the two cartridges. The US Army Picatinny Arsenal stated that the new M855A1 offers improved hard target capability, more consistent performance at all distances, enhanced dependability, improved accuracy, reduced muzzle flash, and higher velocity compared to the M855 round. Further, the Army stated the new M855A1 ammunition is tailored for use in M4 carbines, but should also give enhanced performance in M16 rifles and M249 light machine guns. The new 62-grain (4 g) projectile or bullet used in the M855A1 round has a copper core with a 19-grain (1.2 g) steel "stacked-cone" penetrating tip. The M855A1 cartridge is sometimes referred to as "green ammo" because it fires a lead free projectile.[69][70][80][81][82][83] It is not necessarily more lethal than the M855, but performs more consistently every time it hits a soft target and retains its performance at longer distances. The EPR can penetrate a 3⁄8 in (9.5 mm) thick steel barrier from an M4 at 350 meters and from an M16 at 400 meters. Ballistics for both rounds are similar and don't require weapons to be re-zeroed, but if they are the EPR can be slightly more accurate. The steel-tip penetrator of the M855A1 is noticeably separated from the jacket of the bullet and can spin, but this is part of the design and does not affect performance. The M855A1 costs only 5 cents more per round than the M855.[84] The M855A1 bullet has a 1⁄8 in (3.2 mm) greater length than the M855.[85] Because steel and copper are less dense than lead, the bullet is lengthened inside the case to achieve the same weight as its predecessor.[5] The longer bullet and reverse-drawn jacket make it more stable and accurate in-flight. Its steel tip is exposed from the jacket and bronzed for corrosion resistance. The tip is serrated and larger than the M855's steel tip. The M855A1's bullet composition, better aerodynamics, and higher proof pressures give it an extended effective range for penetration and terminal performance.[86] While effectiveness at different ranges is increased, the M855A1 does not increase the effective ranges at which weapons are expected to hit their targets. The Enhanced Performance Round was made to nearly match the trajectory of the M855 to aid in training consistency - the SS109/M855 ballistic coefficient (G7 BC) of 0.151 was improved to 0.152 for the M855A1[87] - but the ranges to get desired effects are greatly extended.[88]

The M855A1 was put on hold in August 2009 due to the experimental bismuth-tin alloy core exhibiting undependable ballistics at high temperatures. The US Army has since replaced the bismuth-tin alloy core with one of solid copper eliminating the heat issue. The United States Marine Corps purchased 1.8 million rounds in 2010, with plans to adopt the round to replace the interim MK318 SOST rounds used in Afghanistan when the M855A1 project was delayed.[89]

On a media day at Aberdeen Proving Ground on May 4, 2011, reports were given about the M855A1's performance in the field since it was issued 11 months earlier. One primary advantage given by the round is its consistent performance against soft targets. While the older M855 was yaw-dependant, which means its effectiveness depends on its yaw angle when it hits a target, the M855A1 delivers the same effectiveness in a soft target no matter its yaw angle. The new SMP-842 propellant in the round burns quicker in the shorter M4 carbine barrel, ensuring less muzzle flash and greater muzzle velocity. The M855A1 was able to penetrate 3⁄8 inch (9.5 mm) of steel plate at 300 meters. The round even penetrated concrete masonry units, similar to cinder blocks, at 75 meters from an M16 and at 50 meters from an M4, which the M855 could not do at those ranges. Its accuracy is maintained and sometimes increased, as it was able to shoot a 2-inch group at 600 meters. February 2011 was the first time the M855A1 was used more than the M855, and approximately 30 million M855A1 rounds have been fielded from June 2010 to May 2011.[90][91]

The M855A1 was put to the test at the 2012 National Rifle Association's National High-Power Rifle Championship at Camp Perry, Ohio in August 2012. The shooter for the Army was Rob Harbison, a contractor supporting small caliber ammunition capability development at Fort BenningGeorgia. This was a special event for the Project Manager for Maneuver Ammunition Systems and the Army's Maneuver Center of Excellence as it was an opportunity to showcase the capabilities of the Enhanced Performance Round. With an M16 loaded with M855A1 ammo, Harbison fired a perfect 200 points in the Coast Guard Trophy Match, which is 20 shots fired from the sitting position at 200 yards, finishing 17th out of 365 competitors. He also scored a perfect 100 on the final string of ten shots during the Air Force Cup Trophy Match, fired at 600 yards from the prone position, which is 10 shots in a row within the 12-inch, 10-point ring at 600 yards with combat ammunition. Harbison was happy with the performance of the EPR, with his scores showing that the Army's newest general purpose round is accurate enough to go toe-to-toe in the competition with the best ammo that can be bought or hand-loaded. Harbison even said, "I don't think I could have scored any higher if I was using match-grade competition ammunition."[92] It should be noted that the M855A1 was not fired from 1:7 in rifled barrels used in standard Army rifles, but special Army Marksmanship Unit (AMU) match-grade 1:8 in rifled barrels, which produce more accurate results when firing 62-grain rounds.[85]

From fielding in June 2010 to September 2012, Alliant Techsystems delivered over 350 million M855A1 Enhanced Performance Rounds.[93]

Since its introduction, the M855A1 has been criticized for its propellant causing increased fouling of the gun barrel. Post-combat surveys have reported no issues with the EPR in combat. A series of tests found no significant difference in fouling between the old M855 and the M855A1. However, manufacturers have reported "severe degradation" to barrels of their rifles using the M855A1 in tests.[94] The Army attributes pressure and wear issues with the M855A1 to problems with the primer, which they claim to have addressed with a newly designed primer.[95] It uses a modified four-pronged primer anvil for more reliable powder ignition,[86] with a stab crimp rather than a circumferential crimp to better withstand the new load’s higher chamber pressure,[85] increased from 55,000 psi (379.2 MPa) to 62,000 psi (427.5 MPa).[50] During Army carbine testing, the round caused "accelerated bolt wear" from higher chamber pressure and increased bore temperatures. Special Operator testing saw cracks appear on locking lugs and bolts at cam pin holes on average at 6,000 rounds, but sometimes as few as 3,000 rounds during intense automatic firing. Firing several thousand rounds with such high chamber pressures can lead to degraded accuracy over time as parts wear out; these effects can be mitigated through a round counter to keep track of part service life. Weapons with barrel lengths shorter than the M4 firing the M855A1 also experience 50 percent higher pressures than a full-length M16 rifle barrel, which can cause port erosion that can boost the automatic fire rate, increasing the likelihood of jams.[85]

From June 2010 to June 2013, issuing of the M855A1 Enhanced Performance Round removed 1,994 metric tons of lead from the waste stream. 2.1 grams (32 gr) of lead are eliminated from each M855A1 projectile.[96]

The Mk 262 is a match quality round manufactured by Black Hills Ammunition made originally for the Special Purpose Rifle (SPR). It uses a 77-grain (5.0 g) Sierra MatchKing bullet that is more effective at longer ranges than the standard issue M855 round.

In 1999, SOCOM requested Black Hills Ammunition to develop ammunition for the Mk 12 SPR that SOCOM was designing. For the rifle to be accurate out to 600 yards, Black Hills "militarized" a cartridge that used the Sierra 77 grain OTM (Open Tip Match) projectile; it switched from a .223 Remington to 5.56 mm case, increased pressure loading, crimped and sealed the primer, and added a flash retardant to the powder. The Mk 262 Mod 0 was adopted in 2002. Issues came up in development including reliability problems in different temperatures and when the weapon got dirty, and cycling issues in cold weather due to the slightly shorter barrel of the SPR compared to the full-length M16A2 barrel. The problems were addressed with a slower burning powder with a different pressure for use in the barrel, creating the Mk 262 Mod 1 in 2003. During the product improvement stage, the new propellant was found to be more sensitive to heat in weapon chambers during rapid firings, resulting in increased pressures and failure to extract. This was addressed with another powder blend with higher heat tolerance and improved brass. Also during the stage, Black Hills wanted the bullet to be given a cannelure, which had been previously rejected for fear it would affect accuracy. It was eventually added for effective crimping to ensure that the projectile would not move back into the case and cause a malfunction during auto-load feeding. Although the temperature sensitive powder and new bullet changed specifications, the designation remained as the Mod 1.[97]

According to US DoD sources, the Mk 262 round is capable of making kills at 700 meters. Ballistics tests found that the round caused "consistent initial yaw in soft tissue" between 3 and 4 in at ranges from 15 feet to 300 meters. Apparently it is superior to the standard M855 round when fired from an M4 or M16 rifle, increasing accuracy from 3–5 minutes of angle to 2 minute of angle. It evidently possesses superior stopping power, and can allow for engagements to be extended to up to 700 meters when fired from an 18-inch barrel. It appears that this round can drastically improve the performance of any AR-15 platform weapon chambered to .223/5.56 mm. Superior accuracy, wounding capacity, stopping power and range power has made this the preferred round of many special forces operators, and highly desirable as a replacement for the older, Belgian-designed 5.56×45mm SS109/M855 NATO round. In one engagement, a two-man special forces team reported 75 kills with 77 rounds.[98][99] The Mk 262 has a higher ballistic coefficient than the M855 of (G7) 0.181, meaning it loses less velocity at long-range.[87]

Following early engagements in Afghanistan and Iraq, U.S. Special Operations Forces reported that M855 ammunition used in M4A1 rifles was ineffective. In 2005, the Pentagon issued a formal request to the ammunition industry for “enhanced” ammunition. The only business that responded was the Federal Cartridge Company, owned by Alliant Techsystems. Working with the Naval Surface Warfare Center Crane Division, the team created performance objectives for the new ammo: increased consistency from shot to shot regardless of temperature changes, accuracy out of an M4A1 better than 2 minute of angle (2 inches at 100 yards, 3.9 inches at 300 yards), increased stopping power after passing through “intermediate barriers” like walls and car windshields, increased performance and decreased muzzle flash out of shorter barrel FN SCAR rifles, and costs close to the M855. The first prototypes were delivered to the government in August 2007. Increased velocity and decreased muzzle flash were accomplished by the type of powder used. The design of the bullet was called the Open Tip Match Rear Penetrator (OTMRP). The front of it is an open tip backed up by a lead core, while the rear half is solid brass. When the bullet hits a hard barrier, the front half of the bullet smooshes against the barrier, breaking it so the penetrating half of the bullet can go through and hit the target. With the lead section penetrating the target and the brass section following, it was referred to as a "barrier blind" bullet.[37][100]

Officially designated the Mk318 Mod 0 "Cartridge, Caliber 5.56mm Ball, Carbine, Barrier", and called SOST (Special Operations Science and Technology) ammunition, the 62-grain bullet fragments consistently, even out of a 10.5 in barrel. The lead portion fragments in the first few inches of soft tissue, then the solid copper rear penetrates 18 in of tissue (shown though ballistic gelatin) while tumbling. Out of a 14 in barrel, the Mk318 has a muzzle velocity of 2,925 fps.[37][100]

In February 2010, the U.S. Marine Corps adopted the Mk318 for use by infantry. To be fielded by an entire branch of the military, the round is classified as having an "open-tip" bullet, similar to the M118LR 7.62 NATO round. The SOST bullet uses a “reverse drawn” forming process. The base of the bullet is made first, the lead core is placed on top of it, and then the jacketing is pulled up around the lead core from bottom to tip. Conventional, and cheaper, bullets are made with the method of the jacket drawn from the nose to an exposed lead base. The reverse drawn technique leaves an open tip as a byproduct of the manufacturing process, and is not specifically designed for expansion or to affect terminal ballistics. The Pentagon legally cleared the rounds for Marine use in late January. The Marines fielded the Mk318 gradually and in small numbers. Initial studies showed that insurgents hit by it suffered larger exit wounds, although information was limited. SOST rounds were used alongside M855 rounds in situations where the SOST would be more effective.[37][100][101] In July 2010, the Marines purchased 1.8 million M855A1 Enhanced Performance Rounds, in addition to millions of Mk318 rounds in service, as part of its effort to replace its M855 ammo.[102] As of May 2015, Marine combat units still deploy with a mixture of both SOST and M855 rounds.[103]

As the issue of environmentally friendly ammo grew, the Marines looked to see if the Mk318's lead could be replaced while still meeting specifications. They found that by replacing the lead with copper and slightly stretching the jacket around to crimp the nose even more, the bullet's ballistic coefficient increased. To avoid visual confusion with the Mk 262 round, the bullet was entirely nickel-plated for a silver color; the enhanced silver-colored copper jacketed, open tip match, 62-grain projectile was named the Mk318 Mod 1. The Marine Corps will make a decision as whether to field the Mk 318 Mod 1 or M855A1 as its standard rifle round.[104]

The 5.6mm Gewehr Patrone 90 or GP 90 (5.6 mm Rifle Cartridge 90), is the standard round used by the Swiss military in its rifle, the SIG SG 550. The cartridge is also known as the Cart 5,6mm 90 F (French > Cartouche pour Fusil / Italian > Cartuccia per Fucile) to the French- and Italian-speaking Swiss militiamen. The Swiss refer to the round as the 5.6 mm Gw Pat 90, although it is interchangeable with the 5.56×45mm NATO and .223 Remington round. The Gw Pat 90 round firing a 4.1 g (63 gr) FMJ bullet is optimized for use in 5.56 mm (.223 in) caliber barrels with a 254 mm (1:10 in) twist rate.

The Gw Pat 90 was designed for the SIG SG 550 when it came into production in 1987, replacing the SIG SG 510. Previous experience of a change in standard rifle had proved that changing the distance of fire for the training ranges was more expensive than the design of a new ammunition; this prompted the design of a cartridge nominally capable at 300 meters. The cartridge was also designed to reduce pollution by controlling lead emissions.[105] The bullet was originally clad with a nickel alloy jacket, however, this was found to cause excessive barrel wear, so in 1998 the nickel jackets were replaced with tombac jackets. In addition, in 1999 a copper plug was added to the base of the bullet to address environmental concerns.[105]

The ammunition is currently (2009) produced by RUAG Ammotec, a subsidiary of the RUAG group.[106] The ammunition is manufactured in three variations: the standard FMJ round, the tracer round, and a blank round.

The FMJ cartridge has a Copper-Zinc alloy case and uses a double base propellant. The bullet is a 4.1 g (63 gr) tombac jacketed FMJ projectile with a G1 ballistic coefficient of 0.331 (ICAO) / 0.337 (Army Metro). The projectile contains approximately 95% Pb, 2% Sb, 3% Cu, and was designed for terminal ballistic instability. The required accuracy for Gw Pat 90 ammunition out of factory test barrels is 63 mm (0.72 MOA) for 10 rounds (100% radius measurement method) out to 300 m. The Gw Pat 90 cartridge dimensions are in accordance with the civilian C.I.P. standards for the .223 Remington C.I.P. chambering.[107]

The Gw Pat 90 is used both in the Swiss military and in sport shooting. The very high level of individual training in the Swiss militia (every single soldier bearing a weapon has to shoot for qualification once a year; see Gun politics in Switzerland) and the overall use of the Gw Pat 90 by the many Swiss citizens who shoot in competitions and for amusement has resulted in significant input on its usage. Over 1 billion cartridges had been produced as of 2005[update].